Due to characteristics inherent in their manufacturing processes, various types of pipes will have a sidewall thickness that varies as a function of position about the pipe circumference. For example, pipe having a welded seam has a sidewall that is thicker at the weld bead than at positions away from the bead. Seamless pipe and cast pipe made from ductile iron are well known to have sidewalls which vary in thickness around the circumference between a thicker zone and a thinner zone due to an eccentricity between the inner and outer diameters. Cast pipe may also have a cement lining which is sprayed onto the pipe's inner surface. The cement coating is nominally ⅛ to 3/16 inches thick, but will vary in actual thickness due to various process and environmental parameters. A coating of tar, whose thickness may also vary, is often applied to protect the cement. The variation in thickness of the coating layers combines with the variation in pipe wall thickness, sometimes compensating for and sometimes adding to the pipe sidewall variation to produce a pipe having thick and thin sidewall regions.
Devices according to the prior art which perform various processes on pipes that require rotating the pipe relative to the device or vice-versa, for example, cutting circumferential grooves in the outer surfaces of pipes, are exemplified by the pipe cutter disclosed in U.S. Pat. No. 3,247,743 to Frost et al. As shown therein, the sidewall of a pipe is captured between a drive roller and two support rollers mounted on pivoting arms. The drive roller engages the inner surface of the pipe, and the support rollers engage the outside surface of the pipe. The support rollers are pivoted toward one another to force the pipe sidewall against the drive roller. Forcing the pipe against the drive roller provides purchase enabling the drive roller to propel the pipe cutter around the circumference of the pipe when the drive roller is turned. Alternately, for short pipe segments, the pipe cutter remains fixed and the pipe is supported on the support rollers and is turned about its longitudinal axis by rotation of the drive roller to form the groove.
If the pipe sidewall had a constant thickness as a function of circumferential position then the force between the pipe sidewall and the drive roller would also be substantially constant for a given setting of the support rollers' position relatively to the drive roller as the drive roller rotated to move the cutting device about the pipe circumference. However, the varying sidewall thickness results in a significant variation in force between the drive roller and pipe sidewall for a given setting of the support roller positions. The pipe sidewall thickness may vary so much that when a thin region is between the drive roller and the support rollers there may not be sufficient purchase between the pipe sidewall and the drive roller to permit the drive roller to move the cutting device around the pipe circumference. In such a situation the cutting device remains stationary even though the drive roller continues to turn. To remedy this problem the technician overseeing the device adjusts the position of the support rollers by pivoting them closer to the drive roller, thereby compensating for the decrease in sidewall thickness and increasing the force between the drive roller and the inner surface of the pipe sidewall. This adjustment to the position of the support rollers causes the drive roller to gain purchase and begin moving the cutting device around the pipe again about its longitudinal axis. However, as the cutting device orbits the pipe, a thicker sidewall region inevitably moves to a position between the support rollers and the drive roller. The force between the drive rollers and the pipe sidewall increases as a result, placing increased stress on the various machine components and requiring greater torque be applied to turn the drive roller. The force may be great enough to permanently yield or break some parts, and also accelerates failures of high stressed components, such as shafts and keys, by fatigue failure due to the cycling of large stress variations caused by cyclical bending loads imposed on a shaft. There is clearly a need for a pipe processing device, for example, a groove cutting device, which can handle pipes having sidewalls of varying thickness without failing due to high stresses or stress reversals which accelerate fatigue failure.